A Patent Document 1 listed below discloses an exhaust gas heat exchanger for exchanging heat between exhaust gas and cooling fluid of an internal combustion engine. As shown in FIG. 20, the exhaust gas heat exchanger 100 disclosed in the Patent Document 1 includes an outer case 101, plural tubes 110 accommodated in the outer case 101, and a pair of tanks 120 and 121 disposed at both ends of the plural tubes 110.
The outer case 101 is provided with a coolant inlet port 102 and a coolant outlet port 103 for coolant (cooling fluid). Coolant flow path 104 is formed inside the outer case 101 and outside the tubes 110. The both ends of the tubes 110 are opened to insides of the tanks 120 and 121, respectively. An exhaust gas inlet port 120a is formed at the tank 120 on one side, and an exhaust gas outlet port 121a is formed at the tank 121 on another side.
The tubes 110 are stacked. As shown in FIG. 21, each of the tubes 110 is formed by two flat members 110a and 110b. An exhaust gas flow path 111 is formed within each of the tubes 110. A fin 112 is disposed in the exhaust gas flow path 111.
As shown in FIG. 22, the fin 112 is made by a corrugated panel having a rectangular outline shape. On each horizontal wall of the fin 112, plural protruded tabs 113 are cut and raised at intervals along an exhaust gas flow direction S. Each of the protruded tabs 113 has a triangle shape, and is protruded so as to inhibit an exhaust gas flow in the exhaust gas flow path 111. Namely, the protruded tabs 113 are protruded in a perpendicular direction to the exhaust gas flow direction S, and inclined against the exhaust gas flow direction S.
The exhaust gas from the internal combustion engine flows through the exhaust gas flow path 111 in each of the tubes 110. The coolant flows through the coolant flow path 104 in the outer case 101. The exhaust gas and the coolant exchange heat via the tubes 110 and the fin 112. At this heat exchange, the exhaust gas flow is agitated by the protruded tabs 113 of the fin 112, and thereby the heat exchange is facilitated.
As shown in FIG. 23, since the exhaust gas cannot flow straight due to the protruded tab(s) 113, a low pressure area is generated just downstream of the protruded tab 113. As shown in FIGS. 24(a) and (b), the exhaust gas that hits the protruded tab 113 flows over inclined sides 113a and 113b, and then flows around behind the protruded tab 113. Since the protruded tab 113 has a triangle shape, in a first flow flowing over the inclined side 113a and a second flow flowing over the inclined side 113b, flow amounts at upper portions of inclinations of the inclined sides 113a and 113b become large and flow amounts at lower portions of the inclinations become small, respectively, due to the inclinations of the inclined sides 113a and 113b. 
These flows having the above flow amount distribution are drawn into the above-explained low pressure area, and thereby rotating forces act on the first flow and the second flow. As a result, as shown in FIGS. 24(a) and (b), the first flow and the second flow become swirl flows, respectively. In this manner, the two swirl flows are generated downstream of the protruded tab 113. Since these swirl flows break laminar flows near inner surfaces of the exhaust gas flow path 111 and thereby agitate the exhaust gas flow, heat exchange efficiency is improved.